Apparatus for producing electrical oscillations



Oct. 1, 1946. w, WELLS I 2,408,589

APPARATUS FOR PRODUCING ELECTRICAL OSCILLATIONS Filed May 15. 1942 3Sheets-Sheet 2 RESISTOR PHu'ru ELECTRIC csu.

7 noun/322R a :Q 7"" \l\/ fl Q DLAVIER KEY 41 R 42 F196 lF'HUTEIvoL-rAuc TYPE. FELL /46 'F l +9 5 9$ a.

Jnucnlot fllvijz/ W Wells.

Oct; 1946- A; w. WELLS 2,408,589

APPARATUS FOR PRODUCING ELECTRICAL dscIL ATIoNs Filed May 13. 1942 3Sheets-Sheet 5 C 3mm 7 fllviiz/ 14 Weds.

Patented Oct. 1, 1946 APPARATUS FOR PRODUCING ELECTRICAL OSCILLATIONSAlvin W. Wells, Chicago, Ill., assignor of one-half to Jewel W. Wells,Decatur, Ill.

Application May 13, 1942, Serial No. 442,804

3 Claims. 1 The present invention relates to a system or method andapparatus for the production of'clectrical oscillations, and relatesmore particularly to an electrical system which uses as a source ofelectrical oscillations, a plurality of glow discharge oscillatorcircuits.

It is an important object of my invention to provide a method andapparatus for'producing electrical oscillations as obtained from one ormore glow discharge oscillators which may be advantageously andeconomically used in the manufacture of an electric musical instrument,such as an electric organ.

While the invention is hereinafter described in connection with amusical instrument, it will be understood that the inventioncontemplates providing a method and apparatus for the production ofelectrical oscillations as obtained from one or more glow dischargeoscillators which may be capable of general application.

It is well known that a glow discharge oscillator may be employed toproduce electrical oscillations varying in frequency from less than 1cycle per second to more than 30,000 cycles per second. It has beenattempted to employ such a glow discharge oscillator to produceelectrical oscillations of audio frequencies capable of being used in amusical instrument. Great difliculty has been encountered, however, whenattempting to use a plurality of glow discharge oscillators to produce acomplex audio oscillating signal, such as would be obtained when playingtwo or more notes simultaneously in the production of a harmonicpassage. To produce a harmonic 'pa'ssage, it was necessary toelectrically connect two or more oscillator circuits to an outputamplifier and speaker. The difiiculty encountered was apparently due tothe fact that the electrical coupling or connection between two or moreglow discharge oscillators would usually cause the oscillators to tie intogether and behave as a single oscillator. That is to say, theindividual frequencies of the individual glow discharge oscillators,when electrically coupled together, would be degenerated to a singlenewfrequency and would not be a,

complex frequency resulting from the simultaneously superposi'ng of thetwo individual frequencies upon each other. Therefore, it'was apparentthat two or mor glow discharge oscillators could not be electricallycoupled together at the same time to produce a harmonic passage becauseof the fact that the specific frequencies of the individual oscillatorswere changed.

Attempts have been made to overcome this difiiculty by employingseparate oscillation or buffer amplifiers for each glow dischargeoscillator to thereby separate the electrical circuits of eachoscillator from one another when two or more of the oscillators aresimultaneously connected or keyed to the output. Since the electricalinstrument may contain from sixty-one to several hundred oscillators, inorder to afford a satisfactory tonal range, the practice of usingseparate oscillation or buffer amplifiers for each oscillator wouldrender the musical instrument very expensive and complicated tomanufacture.

By my invention, I have provided a method and apparatus for coupling twoor more glow discharge oscillators simultaneously to an output circuitwithout affecting the individual oscillation frequency of the separateglow discharge oscillators. I have provided a photo-electric systemwhich will be responsive to the glow or fluctuating light emitted by theglow discharge tube of the oscillator circuit. The light from the glowdischarge tube fluctuates at the exact frequency of the oscillatingcircuit. By optically coupling the glow discharge oscillator to thephoto-electric system and associated amplifier for reproducing theelectrical signal into audible sound, I have provided a system wherebytwo or more glow discharge oscillators may be connected at the same timeto the output without affecting the individual frequencie of thoscillator circuits. This photo-electric system of coupling the glowdischarge oscillators to an output circuit is very economical tomanufacture.

It is, therefore, a further object of my invention to provide a methodand apparatus of optically coupling two or more glow dischargeoscillators to a photo-electric reproducing circuit for translating theoscillation frequencies generated in the oscillator circuits to audiblefrequencies.

It is a further object of my invention to provide a method of producinga complex electrical oscillation resulting from the effect of asimultanecus exposure of two or more fluctuating light sources asobtained from two or more glow discharge oscillators upon aphoto-electric reproducing system.

It is still a further object of my invention to provide a musicalinstrument employing the optical system of coupling the glow dischargeoscillators to the photo-electric reproducing system having meansassociated with a key board for selectively exposing each of the glowdischarge oscillators to' the photo-electric reproducing system.

It is another object of my invention to provide' a musical instrumentemploying the optical- 3 system of coupling the glow dischargeoscillators to the photo-electric reproducing system having meansassociated with a key board for selectively exposing each of the glowdischarge oscillator tubes to the photo-electric element and soconstructed that the amount of optical exposure of the glow dischargetube to the photo-electric element will be proportional to the amount ofpressure applied to the clavier key of the key board, thus offering ameans of varying the dynamics of the individual electrical signals andenabling the production of percussion effects, or the like.

It is still a further object of my invention to provide a musicalinstrument employing the optical system of coupling the glow dischargeoscillators to the photo-electric reproducing system in which the glowdischarge oscillator circuits are provided with a means for periodicallyraising and lowering the oscillation frequencies to thereby produce avibrato effect.

Further objects and advantages of my invention will be apparent byreference to the follow ing specification and drawings, in which:

Figure 1 is a schematic diagram of a conventional glow dischargeoscillator using a neon glow tube;

Figure 2 is a schematic diagram of a glow discharge oscillator inassociation or optically cou pled to a photo-electric reproducing systemcomprising a photo-electric cell and a photo-electric amplifier ofstandard design; Figure 3 is a schematic diagram of a number of glowdischarge oscillators provided with a con:- mon source of appliedvoltage and showing a means for periodically raising and lowering theapplied voltage to provide a tremolo or vibrato effect;

Figure 4 is a diagrammatic showing of one form of apparatus forselectively exposing the glow 4 discharge tube of an oscillator circuitto a photoelectric cell by means of an optical shutter in connectionwith a clavier key;

Figure 5 is a schematic diagram showing a plurality of glow dischargeoscilators and diagrammatically showing a single photo-voltaic typephoto-electric element responsive to the glow of any of the glowdischarge tubes in the circuit;

Figure 6 is a cross-sectional view or the photovoltaic type celldiagrammatically shown in Figure 5;

Figure 7 is a diagrammatic showing of a modified form of key arrangementfor selectively exposing the glow discharge tube of an oscillatorcircuit to a photo-electric cell in which the degree of exposure will beproportional to the amount of pressure on the key or the relativedepressed position of the key;

Figure 8 is a diagrammatic showing of still another modified form of keyarrangement to i provide pressure responsive dynamic control of theindividual tones produced; and

Figure 9 is a front detail view of the optical shutters shown in Figure8.

Referring to the drawings, Figure 1 is a scheage, cause the circuit tooscillate, and the frequency of the oscillation may be varied byadjusting the constants of the circuit, such as the resistor 2 and thecondenser 4. The frequency of the oscillation produced is aiTected bythe capacity, resistance, and applied voltage in the circuit. Therefore,the glow discharge oscillator circuit may be tuned to differentoscillation frequencies by varying the value of any of these constants,such as the capacity, resistance, and applied voltage. If the voltage ismaintained constant, the condenser or resistor values may be varied tochange the oscillation frequency. I have found, by experimentation, thatthe best practice for tuning the oscillator to desired frequencies is toadjust the values of the resistor 2 for the lower frequencies includedin the range up to 1,000 cycles per second and to adjust the value ofthe condenser 4 for the higher frequencies included in the range of1,080 cycles per second up to the highest frequency desired.

In Figure 2 of the drawings, I have shown a schematic diagram for a glowdischarge oscillator that is optically coupled to a photo-electricreproducing system. The glow discharge tube 5 is connected in serieswith the resistor 6 and the applied voltage as obtained from the voltagedi vider l. A condenser 8 is shunted in parallel with the resistance 6.By adjusting the resistance 6 or the condenser 8 as described above inconnection with Figure l, the frequency of 0*- cillation in theoscillator circuit may be changed. It is a function of the glowdischarge oscillator tube 5 that the light or glow emitted by the tubewhen oscillating in an oscillating circuit will flue-- tuate at afrequency which is identical to the frequency of the oscillatingcircuit. Optically conected to the glow discharge tube 5 is aphotoelectric cell or other photo-electric responsive element 9. Thisphoto-electric cell is provided with a suitable photo-electric amplifierwhich I have shown in Figure 2 of the drawings to be of the resistancecoupled type. Plate, grid, and bias resistors R, together with couplingand by-pass condensers C, triode tube Hi, and output transformer T,constitute a schematic showing of a standard type of resistance coupledamplifier. It should be understood that any other desired form ofamplifier, such as a transformer-coupled or a direct-coupled amplifier,may be used in place of the resistance-coupled amplifier which I haveshown. It will be apparent that any desired number of photo-electriccells may be connected to a single amplifier.

The operationof the electrical system shown in Figure 2 of the drawingsand described above is as follows: When the oscillating circuit isenergized, th glow discharge tube 5 will emit a fluctuating lightcorresponding in frequency to the frequency of the oscillating circuitin which the glow discharge tube is connected. The fluctuating lightemitted by the glow discharge tube 5 is directed upon the photo-electricelement 9 and will cause the photo-electric element 9 to translate thelight impulses into a fluctuating or oscillating current which will becoupled to the photo-electric circuit by the resistance-coupledpreamplifier and transformed by the output transformer T to the properimpedance desired for a power amplifier or th voice coil of aloudspeaker (not shown). Thus, an electrical system is provided forcoupling a glow discharge oscillator to a reproducing system in whichthe circuit for the glow discharge oscillator is electricallyindependent of the circuit for the reproducing c resses r system. Withsuch an arrangement of optical coupling, it is possible to connect two.or more glow. discharg oscillators simultaneously to a photo-electricreproducing system, and. because the circuits of the oscillators and thecircuit of. the reproducing system are independent, there will be noelectricalcoupling or connection between the. individual oscillatorcircuits, and the difliculties previously encountered, wher electricalcoupling or connection between the oscillators was used,.are avoided.

Figure 3 of the drawings shows a plurality of glow. dischargeoscillators provided with a common source of applied voltage. Theappliedvoltage for the circuit. shown in Figure 3 is, periodicallyvariedabout a mid-point to obtain a period: ie rise and fall in the frequencyof oscillations produced in the glow discharge oscillator circuits. Avoltage divider generally indicated by D. is divided into resistancesegments ll, [2, l3, and M. An appliedvoltage of 350 volts is suppliedto the voltage divider D. At the point between the resistances. H and12, the switch contact I5 is connected, and at the point between theresistances l2 and IS, the glow discharge oscillator are connected bymeans of the line l6. At. the point between resistances l3 and M, theswitch contact H is electrically connected. The switch blade l8ispivoted at 19 and is connected by means of the pitman arm 20 to a crank2| on the shaft ofv a small electric. motor 22,. The small electricmotor is preferably of the synchronous type which will run atconstantspeed, or. it may, if desired, .be providedwith suitable meansto vary the speed, and it will be apparentthattheswitch blade l9. willbeperiodically connectedat regular intervals with the switch contacts l5.and I! when the electric motor is inoperation. Theswitch blade I8 ielectrically connectedthrough an on or off switch 23. and resistor 24t0,the line l6 supplyingapplied voltage to. the glow discharge oscillatorcircuits. The glow discharge oscillator circuits which are connected tothe common source ofapplied voltage, as supplied to the line I 6 .andthe ground connection 25, are provided witha number of glow dischargetubes 26,. It will benoted that. some of the glow discharge tubes. 26are provided with variable resistances 21 to adjust the. frequency ofthe oscillating circuit and thereby the frequency of the fluctuatinglight emitted by the discharge tube, while others of the glow dischargetubes are provided with variable. condensers 2B for adjusting thefrequency of. the oscillations and the fluctuating light. As previouslymentioned, the variable resistor. 21 is adjusted in the oscillatingcircuit when the oscillator is tobe tuned to lower frequencies, whereasthe variable condenser 28 is adjusted when tuning the oscillatingcircuit to higher frequencies.

As the oscillation frequency of the glow dis.- charge oscillator is afunction of the resistance capacity and applied voltage, it will beapparent that varying the applied voltage will vary the frequency ofoscillations. As shown in Figure 3, a plurality of glow dischargoscillators are provided with a common source of applied voltage. Anelectrical system is also shown in Figure 3 and described above forperiodically raising and lowering the applied voltage about a mid-point,so asto periodically raise and lower the oscillation frequencies of theoscillator circuits, and thereby produce a vibrato effect which will beaudible when the fluctuating light from the glow discharge tubes isinterpreted by the -photoeelec- 6 tric reproducing system; and speaker.In operation; the electrical. system for periodically raisingand-lowering the applied voltage to the oscillator circuit isas-follows: The; voltage of 350 volts appliedacross the voltage dividerD which is comprised of. the resistance segments ll, l2, l3, and I4.When the vibrating switch blade I8 is in the mid-position shown inFigure 3 of the drawings andis not electrically contacting the switchcontactslfisor H; the applied voltage supplied to the line H5 in theglow discharge oscillators is obtained from the voltage divider D at thepoint between. resistance segments l2 and I3. The oscillators will thenbe supplied with a normal amount of applied voltage. When the vibratingswitch blade I8 is electrically contacting the switch contact I5, theresistance 24 will be placed in parallel with the resistance segment,l2, thus effectively reducing the total resistance of the resistors I land I2. and increasing the applied voltage tothe line l6 and the glowdischarge oscillators. The frequency of the oscillators will then beraised above, the normal. When the vibrating switch blade I8 is inelectrical contact with the switch contact [1, the resistance 24 will beconnected in parallel with the resistance l3, thus effectively reducingthe amount of resistances l3 and I4. As the resistances I3 and I4 arepart of the voltag divider and are connected across the lines Hiv and 25of the applied voltage for the glow discharge oscillators, the appliedvoltage to the line 16 and the oscillators will be reduced. Therefore,the frequency of the oscillators will then be lowered below the normal.Thus, it will be seen that the vibrating switch blade 18, periodicallycontacting the switch contacts l5 and I1, will cause the appliedvoltageto the line I S and the glow discharge oscillators to be periodicallyraised and lowered about a normal. When the switch blade i8 is in theposition shown in Figure 3 and not contacting the electrical contacts 15and I1, the applied voltage will be at a midpoint because of the factthat the resistance 24 is not electrically connected-into the circuit.It is important that the applied voltage common to all of th oscillatorcircuits be varied above and below a normal or mid-point as thisprevents th illusion of the musical instrument'going out of tune whenthe vibrato is turned on or oif. A fixed condenser 29 is shown acrossthe output of the voltage varying device to eliminate any undesirableoscillations in the glow discharge oscillators, due to the sparking ofthe switch contacts I5 and H with the vibrating switch blade l8.

In Figure 4, I have diagrammatically shown oneform of apparatus forselectively exposing the fluctuating light from the glow tubes to aphotoelectric element. Mounted within the lightproof box 30 is thephoto-electric cell 31 and neon glow tube or its equivalent 32. Thecondenser 33 and resistor 34 for the glow discharge oscillator circuitare also mounted within the lightproof box. A thimble 35 encloses theglow tube 32 and is provided with an aperture or window 36.Positioned'in front of the window 36 i a shutter element 31 pivoted onthe spring arm 38 bolted to the lightproof box at 39. A connecting rodin is connected to the spring arm 38 of the shutter 31 and passesthrough the lightproof box to connect with the'clavier key 4| mountedupon the spring arm 42. When the clavier key 4| is depressed, as shownin the dotted lines, the shutter 31 will be removed from in front of thewindow 36, allowing the fluctuating light from the glow discharge tube3.2 toimpinge upon the elements of the photo-electric cell 3|. With thisform of apparatus, it is possible to maintain the glow dischargeoscillators in operation at all times. Whenever it is desired tooptically couple a glow discharge oscillation circuit to thephoto-electric reproducing system, the clavier key is depressed, thusremoving the shutter from in front of the aperture and permitting thefluctuating light from the glow discharge tube to affect thephotoelectric element. Any number of lightproof boxes containing thephoto-electric tube, together with the glow discharge tube and shutter,may be provided and simultaneously operated, if desired, withoutaffecting the individual oscillation frequencies of the individual glowdischarge oscillators, since the electrical circuits for thephotoelectric reproducing system and the glow discharge oscillators areindependent of one another.

In Figure 5 of the drawings, I have shown a number of glow dischargeoscillator circuits provided with glOW tubes 43 and supplied with acommon source of applied voltage of 350 volts. Variable resistors 44 andvariable condensers 45 are used to adjust the frequency of thefluctuating light given oil by each of the glow discharge tubes 43. Aphoto-voltaic type of photo-electric element 46 is provided to receivethe fluctuating light of the glow tubes 43. The photo-voltaic type ofphoto-electric cell is an elongated tube which can be made of sufiicientlength to simultaneously respond to a large number of glow dischargetubes. If the glow discharge oscillator circuits and tubes are to beused in the construction of a musical instrument, such a an electricorgan, it will be necessary to have at least ninetysix glow dischargetubes and associated oscillator circuits. These glow disharge tubes maybe placed in a line within the cabinet of the musical instrument, andthe elongated photo-voltaic type of photo-electric cell may be opticallyaligned with the line of glow discharge tubes. If de sired, however,more than one photo-voltaic type of photo-electric cells may be used totake care of all of the oscillator circuits and glow discharge tubesnecessary to produce the ordinary range of musical notes on a keyboardinstrument.

A cross-section of the photo-voltaic type of photo-electric cell isshown in Figure 6. An elongated casing 41 is provided with a glasswindow 48. Positioned within the casing is a surface of light-sensitivesubstance 49 deposited on a metal plate 59. An electrical contact orbinding post 5| is connected to the metal plate 50, while anotherelectrical contact or binding post (not shown) is connected to thelight-sensitive surface 49. The electrical.,contacts of the photovoltaictype of photo-electric cell are connected in the usual manner to asuitable amplifier.

In Figures 7, 8, and 9 of the drawings I have shown two modified formsof keying arrangements for selectively exposing the light of a glowdischarge tube to a, photo-electric element. The use of an opticalkeying system such as my invention employs offers a means for varyingthe dynamic volume of the individual electrical oscillations produced bythe photo-electric reproducing system in accordance with the manner inwhich the keys are struck. That is to say, I am enabled toproportionally expose the glow discharge tube to the photo-electricelement in accordance with the pressure applied and the position of themanually depressible key to thus give a greater dynamic volume to theelectrical oscillation or tone produced in response to a greaterpressure applied to depress the clavier key.

Referring to Figure 7, a clavier key 52 mounted upon the spring arm 53is connected by the connecting rod 54 to the shutter 55 pivoted on thespring arm 53. A thimble 5i encloses the glow discharge tube 58 and hasan aperture 59 through which the light from the glow discharge tube maypass to the photo-electric cell 69. When the clavier key 52 is depressedthe shutter 55 will be removed from in front of the aperture 59, thuseyposing the glow discharge tube 58 to the photoelectric cell 60. Itwill be apparent that the amount of exposure or uncovering of theaperture 59 will be proportional to the pressure applied to depress theclavier key 52. To indicate the relative depressed position of theclavier key 52, I have provided stops El, 62, and 63 cooperating,respectively, with spring 54, 65, and 66. If the key 52 is depressedslightly, stop 6i will contact sprin 64, thus giving an indication ofthe position of the key 52 and the relative exposure of the aperture 59.If greater pressure is applied to depress the key 52 the stop 62 willengage spring 65, thu giving a further indication of the position of theclavier key and the relative exposure of the aperture 59. A rest 57 isprovided to indicate to the player when the clavier key has beendepressed all of the way to completely expose the aperture 59, thusgiving the greatest dynamic volume to the electrical oscillationreproduced through the photo-electric system.

By skillfully playing, it is possible, with the form of my inventionjust described above, to accent the individual notes in a passage to anydegree desired, producing various musical effects, such as percussion orpizzicato effects.

Referring to Figures 8 and 9, a slightly modified key arrangement isshown for the application of my invention to an instrument primarilydesigned for producing percussion effects by proportionally exposing theglow discharge tube to the photo-electric cell in response to thepressure applied to the key. The clavier key 68 is pivoted at 69 and isprovided with a stepped arrangement of stop positions 10, ll, and i2.Springs l3, l4, and 15 are adapted to cooperate, respectively, with thestop positions Iii, II, and 12. A rest 11 is provided for indicating thefully depressed position of the key 68. A shutter element [8 having anaperture or window 19 is slidably positioned for vertical movementwithin the light proof box and in front of the aperture or window 8| ofthe thimble 82 enclosing the glow discharge tube 83. A detailed frontview of the shutter element 18 is shown in Figure 9 of the drawings. Aweight 84 is positioned at the top of the shutter element 18. When thekey 68 is struck the lever end 85 of the key will cooperate with thebottom end of the shutter element 18 to thrust said element upward, andthe intensity of the impact will determine the upward distance which theshutter 78 is moved. If the key 68 is struck sharply the shutter element18 will be thrust upward to its full extent, thus completely exposingthe glow discharge tube to the photo-electric cell. When the key isfully depressed, as shown by the dotted lines A, the shutter element 18will be moved so that the center line C of the aperture 19 will be inthe position shown at A to completely register the shutter 19 with thewindow or aperture 8| of the thimble 82. If the key 68 is depressed tothe position shown in dotted lines at B the center line of the aperture19 will be in the position shown at B, thus partially exposing the lightof the glow tube to the photo-electric element. By skillfully playingwith this arrangement the key 68 may be struck sharply tomomentarilycompletely expose the glow discharge tube, but due to the weight 84 andas soon as the momentum of the shutter element I8 is overcome theshutter will fall to again shield the light of the glow discharge tubefrom the photo-electric cell, thus producing a percussion tone of greatinitial dynamic volume, but decaying to a tone of low volume or of novolume at all. If the player desires, the key 68 may be struck sharplyto the position shown in the dotted lines at A, and then be released tothe position shown in dotted lines at B, thus momentarily completelyexposing the glow discharge tube, but subsequently allowing the glowdischarge tube to be but partially exposed. This is of great importancewhen the player desires to cause the volume of the tone to be of greatinitial intensity but to decay gradually to a lower volume level andthus prevent an abrupt ending of the tone.

I have described above a system for producing electrical oscillations inwhich a plurality of glow discharge oscillator circuits may be used. Byoptically coupling the individual glow discharge oscillator circuits tothe reproducing system, I am enabled to couple two or more of theindividual oscillator circuits to a single reproducing system withoutelectrically connecting the individual oscillator circuits to eachother. The fluctuating light emitted by the glow discharge tube of theglow discharge oscillator will fluctuate at the exact frequency of theoscillation in the oscillating circuit, and as the photo-electricelement is responsive to this fluctuating light, the electrical signalgenerated in the photo-electric cell will be identical in frequency tothe frequency of the oscillating circuit.

While I have described above, in connection with Figures 4 and 7 through9 of the drawings, mechanical methods of selectively exposing a glowdischarge oscillator to a photo-electric reproducing system, it shouldbe understood that other means of selectively coupling the oscillatorcircuits to the reproducing system may be employed. Instead ofmaintaining the discharge tube and oscillator circuit energized at alltimes and using an optical shutter to control the coupling of the glowdischarge tube to the photoelectric cell, I may provide a switch, suchas is shown in Figure 1 of the drawings, in series with the glow tubeand its oscillator circuit. This switch S, shown in Figure 1, may beconnected to 10 the clavier key M to thus selectively energize thedesired glow tube and oscillator circuit to cause the glow tube to emitthe desired frequency of fluctuating light for afiecting thephoto-electric reproducing system.

It is to be understood that the system shown and described is merely anillustration of the preferred embodiment, and that such changes may bemade as fall Within the purview of one skilled in the art withoutdeparting from the spirit of the invention and the scope of the appendedclaims.

I claim:

1. A light sensitive system comprising a plurality of glow dischargetubes, oscillating circuits continuingly energizing said tubes, meansfor tuning each of said circuits to a certain frequency, electric meansoutside of said oscillating circuits adapted to respond to thefluctuating light emitted by said tubes, means normally shielding saidphoto-electric means from said light, selectively operable means wherebysaid shielding means is rendered ineffective as regards the lightemitted by one or more of said tubes so that said photo-electric meansis correspondingly activated, and an amplifier fed by saidphoto-electric means.

2. A light sensitive system comprising a plurality of glow dischargetubes, oscillating circuits continuingly energizing said tubes, meansfor tuning each of said circuits to a certain frequency, photoelectricmeans outside of said oscillating circuits adapted to respond to thefluctuating light emitted by said tubes, a movable shutter associatedwith each of said tubes normally shielding said photo-electric meansfrom said light, means for selectively moving said shutters to permitactivation of said photo-electric means by the light emitted by one ormore of said tubes, and an amplifier fed by said photo-electric means,

3. A light sensitive system comprising a plurality of glow dischargetubes, oscillating circuits adapted to continuingly energize said tubes,a common normal voltage supply for said circuits, means for tuning eachof said circuits to a certain normal frequency, a photo-electric systeminde pendent of said circuits disposed for energization by light emittedb said tubes, means for selectively controlling such light emission, anamplifier fed by said system and means for rapidly cyclically varyingthe voltage supply between above and below normal whereby to causecorresponding variations in the frequency of said tubes and thus producea vibrato effect.

ALVIN w. WELLS.

